1. Field
This invention relates generally to a wide bandwidth, narrow beam coaxial antenna feed horn and, more particularly, to a wide bandwidth, coaxial antenna feed horn that includes a tapered dielectric at the horn aperture for impedance matching to free space and/or a multi-layered dielectric member that allows propagation of a TE11 sum mode and a TE12 difference mode starting at the same cut-off frequency, where polarization may be linear or circular.
2. Discussion
For certain communications applications, it is desirable to have a broadband system, namely, operation over a relatively wide frequency range, typically greater than 1.5:1. In some reflector based systems, it is desirable to have a feed with a small foot print, making it suitable for illuminating very low focal length to diameter ratios reflector lens.
In certain communications systems, signal tracking between the receiver and transmitter is achieved with the use of a sum and difference pattern. A sum pattern presents a broadside peak radiation pattern, while a difference pattern provides a broadside null radiation pattern. In this case, two electromagnetic propagation modes, the transverse-electric (TE) modes (TE11, TE12) in the feed horn are needed to realize a sum and difference within the same frequency range. In general, the TM00 mode is used for linear polarization. System performance requirements may call for a large instantaneous RF bandwidth and a small physical footprint, to name a few.
A critical element to achieve the signal tracking feature, while meeting system specifications is the feed antenna. To meet size constraints, a smaller aperture size is usually desired, such as that of a coaxial horn antenna. However, its cut-off frequency of the TE12 difference mode is twice the cut-off frequency of the TE11 sum mode, where the cut-off frequency of a particular mode is the lowest frequency that the mode can propagate. It is known in the art to load such a feed horn with a dielectric to lower the cut-off frequency of a particular mode. In addition to realizing the necessary modes for generating the sum and difference mode, ample signal from the feed horn must be transmitted/received. Namely, for a small aperture relative to the operating wavelength feed horn, there exists a significant impedance mismatch between the dielectric and free space resulting in significant signal loss.